A process for the application of the hot wire anemometry principle to follow milk coagulation is described in document EP-A-0-144,443. This method makes it possible more generally to follow a liquid-gel transition under isothermal conditions. In fact, energy is introduced into initially liquid medium in the form of heat by means of a platinum wire, after which a measurement of the temperature of this wire is carried out. In a liquid medium, natural convection produces an equilibrium of heat transfers between the wire and the product so that the wire temperature is constant, slightly higher than that of the surrounding medium. If the medium coagulates or gels, the change in the structure of this medium is accompanied by a change in the thermal regime with a changeover from natural convection to conduction. This is reflected in an increase in the temperature of the platinum wire, which corresponds to a change in the heat transfer coefficient in the medium, since the heat conductivity of the latter remains constant. In the food industry field, firstly, the isothermal state is never perfectly achieved and, secondly, many cases of gelling or of coagulation are coupled with a temperature change, for example the gelling of gelatin, of polysaccharides and, more generally, the manufacture of sauces and of jams, as also is the coagulation on heating of milk to which rennet has been added when cold, etc. In the cases set out above the perturbations introduced by the temperature change do not allow the sensor proposed in the abovementioned patent to display the coagulation phenomenon.